Draw an angled line through the centre of it and edit the line to have a thickness and change it's texture to insulation.
Alternately, put the polygon on a separate layer. Turn off the visiblility of other layers. Make a paint pattern by exploding a insulalion line and creating a custom paint, and paint the polygon with it.
You could also trace the polygon with a shade break and add a custom shade paint.
I have so many insulation paints, but I can't remember if any of them are standard - I don't think they are, but creating a custom paint is simple. For insulation I'd recommend having multiple custom depth paints to suit the insulation thickness that you are going to use.